Phenoxy herbicide molecules - rotatable in 3 dimensions

Herbicides, also known as weedkillers, are quite economically important as they reduce competition between crop plants and weeds. There are several categories, with different modes of action.

This unit concentrates on phenoxy herbicides, which simulate the action of the natural auxin Indole Acetic Acid (IAA).

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Indole Acetic Acid (IAA)

is a plant hormone or growth regulator, well known for causing curvature in roots and shoots, resulting in phototropism and geotropism. It is also called auxin, and there are a number of similar compounds called auxins.

It is used in 'rooting powder' as a stimulant to root growth in cuttings, and it is known to cause growth of undifferentiated cells - 'callus' - in plant tissue culture.

However when sprayed on the outside of dicotyledonous - 'broadleaved' - plants it causes uncontrolled growth resulting in death, but it does not affect monocots in the same way.

The two-ring indole group is also found in the amino acid tryptophan, and acetic acid is also known as ethanoic acid.

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Phenoxyacetic acid

is the basis of a variety of synthetic herbicides ('phenoxy herbicides'), which act by simulating the action of auxins.

The single-ring phenoxy group is here attached to acetic acid is also known as ethanoic acid.

The phenoxy group can be expanded by chlorination.

Phenoxyacetic acid
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2, 4 - D

(2,4-dichlorophenoxyacetic acid)

Developed as a longer-lasting analogue to indole acid acid, it was considered during World War 2 for the purpose of killing crop plants, but it was found that (monocotyledonous) crops such as wheat and other cereals, maize and rice were unaffected by low concentrations whereas broadleaved 'weeds' - dicotyledonous plants - were killed by it.

This enabled the production of herbicide products which increased food production with less need for physical weed control. It was patented in 1945, but this has now expired and large amounts continue to be produced around the world. The estimated use of 2,4-D in US agriculture is 20,000,000 kg per year. Its main use is in the control of broad-leafed weeds in pastures, orchards, and cereal crops, as well as in weed killing before planting legumes (beans, peas, lentils, and chickpeas).

2,4 D
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(2,4,5-trichlorophenoxyacetic acid)

Like 2,4-D, 2,4,5-T was developed in the late 1940s and widely used in the agricultural industry until being phased out, starting in the late 1970s due to toxicity concerns.

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Agent Orange

A mixture of 2,4 D and 2,4,5 T known as 'Agent Orange' was sprayed on large areas of Southeast Asia. There were in fact a number of 'Rainbow herbicides', which were identified by the colour of a stripe painted on the side of 200-litre drums in which they were distributed.

In fact this herbicide mixture has been used in a number of contexts:
  • From the late 1940s in the USA it was sprayed along railways and under power cables to control tree growth
  • During the 'Malayan Emergency' from 1952 to 1960 it was used by the British, by aerial spraying an area of 1250 acres along roadsides to remove cover for 'insurgents' (as well as to reduce crop growth)
  • A large area (31,000 km2) of Vietnam was sprayed from 1961 to 1971 - over 80 million litres were used. It was described as a 'tactical-use chemical': a defoliant to reduce tree cover, and its action as a herbicide against crop plants was not mentioned.
  • More recently it was used in Brazil to clear out sections of the Amazon rainforest for agriculture
It was discovered that Agent Orange was contaminated with small amounts - ranging from parts per billion (ppb) to parts per million (ppm) - of a compound known as dioxin. This undoubtedly adversely affected the health of millions of people on the ground who were sprayed with it, as well as thousands of Americans who were contaminated as they distributed it.


There are a number of similar compounds which form widespread environmental pollutants. They tend to be stored in fatty animal tissue and are chemically very stable so they bioaccumulate in food chains.

In fact it is the central section of this particular molecule that is called dioxin.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)
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Web references

Phenoxy herbicide From Wikipedia, the free encyclopedia

Agent Orange From Wikipedia, the free encyclopedia

Dioxins and their effects on human health - from the WHO